Ethernet Connectivity Fault Management (CFM) is an end-to-end per-service-instance Ethernet layer protocol that includes proactive connectivity monitoring, fault verification, and fault isolation. CFM uses maintenance points. A maintenance point is a demarcation point on an interface (port) that participates in CFM within a maintenance domain. Maintenance points on device ports act as filters that confine CFM frames within the bounds of a domain by dropping frames that do not belong to the correct level. Two classes of maintenance points exist, Maintenance Endpoints (MEPs) and Maintenance Intermediate points (MIPs).
MEPS exist at a per-maintenance domain (level) and service provider VLAN (S-VLAN). MEPs at an edge of a domain define the domain boundary. MEPs within the bounds of a maintenance domain confine CFM messages. When configured to do so, MEPs proactively transmit CFM continuity check messages (CCMs). At the request of an administrator, MEPs transmit traceroute and loopback messages.
MIPs exist at a per maintenance domain (level) and for all S-VLANs enabled or allowed on a port. MIPS are internal to a domain, not at the boundary. CFM frames received from MEPs and other MIPs are cataloged and forwarded. CFM frames at a lower level are stopped and dropped. All CFM frames at a higher level are forwarded.